Enter the total resistance of the conductor and its length into the calculator to determine the ohms per foot. This calculator helps in evaluating the resistance per unit length of an electrical conductor.
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Ohms Per Foot Formula
Ohms per foot describes the resistance a conductor adds for each foot of length. It is a useful way to compare wires, estimate voltage drop, evaluate heating losses, and check whether a measured conductor behaves as expected.
r_{ft} = \frac{R}{L}Where:
- rft = resistance per foot
- R = total resistance of the conductor
- L = conductor length in feet
If you know any two values, you can solve for the third:
R = r_{ft}LL = \frac{R}{r_{ft}}How to Calculate Ohms Per Foot
- Measure or obtain the total resistance of the conductor.
- Measure the conductor length in feet.
- Divide the total resistance by the total length.
- Interpret the result as the resistance added by each foot of that conductor.
If your length is entered in meters or yards, the idea is exactly the same: you are finding resistance per unit length for the selected unit.
Example
If a conductor has a total resistance of 5 ohms and a length of 50 feet, then:
r_{ft} = \frac{5}{50} = 0.1\ \Omega/ftThis means each additional foot of the same conductor adds about 0.1 ohms of resistance.
Why Ohms Per Foot Matters
Resistance per foot directly affects circuit performance. As wire length increases, total resistance increases, which can reduce delivered voltage and increase heat generation.
V_{drop} = IRP_{loss} = I^2R- Higher ohms per foot usually means more voltage drop over long runs.
- Higher resistance also means more power lost as heat at the same current.
- Lower ohms per foot generally indicates a better conducting path for power delivery.
- Comparing conductors is easier when resistance is normalized to a unit length.
What Changes the Value?
Ohms per foot depends mainly on conductor material and cross-sectional area. In consistent units, resistance follows:
R = \rho \frac{L}{A}Dividing by length shows why conductor type and size matter so much:
\frac{R}{L} = \frac{\rho}{A}- Material: conductors with higher resistivity have higher resistance per foot.
- Area: larger cross-sectional area lowers resistance per foot.
- Temperature: the resistance of most metallic conductors increases as temperature rises.
- Condition: corrosion, poor terminations, and damaged strands can raise measured resistance.
Unit Conversion Reference
| Conversion | Formula |
|---|---|
| Ohms per foot to ohms per meter | r_m = 3.28084\,r_{ft} |
| Ohms per meter to ohms per foot | r_{ft} = 0.3048\,r_m |
| Ohms per foot to ohms per yard | r_{yd} = 3\,r_{ft} |
Practical Notes
- Use the same conductor segment for both the resistance and length inputs.
- Keep units consistent before interpreting the result.
- For voltage-drop estimates in a complete circuit, use the total current path length when appropriate rather than only the one-way distance.
- Very small resistance values may require a meter designed for low-resistance measurement to avoid lead and contact errors.
- If a measured value is much higher than expected, inspect for loose connections, damaged wire, or temperature-related changes.
Common Questions
- Is a lower ohms-per-foot value better?
- For most power and wiring applications, yes. Lower resistance per foot means less voltage drop and less heat for the same current.
- Can this be used for any conductor?
- Yes, as long as the total resistance and total length refer to the same conductor and the units are entered correctly.
- Why might my measured value differ from a wire table?
- Temperature, connection resistance, strand construction, conductor condition, and meter accuracy can all affect the reading.
- What does a high result suggest?
- A high ohms-per-foot value usually points to a smaller conductor, a higher-resistivity material, excess temperature, or an abnormal resistance issue that should be checked.
